1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting multicast broadcast service (MB S) data in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference. In a system using the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner.
An electrical and electronics engineers (IEEE) 802.16 wireless communication system provides a high-speed multimedia communication service to users in addition to the conventional voice and packet data communication services. The multimedia communication service is a service for transmitting data packets to a plurality of user equipments (UEs), and is referred to as various terms such as a broadcast/multicast service, a multimedia broadcast multicast service (MBMS), a multimedia and broadcast service (MBS), and a point-to-multipoint service. These terms are used without distinction in the following description. The MBS is based on Internet protocol (IP) multicast and allows UEs to receive the same multimedia data by sharing resources required for data packet transmission. Therefore, resource efficiency can be increased by the MBS.
The MBS can be classified into two types. The first type is a single-BS access type in which a service is provided by one BS in one cell. Examples of the first type include mobile TV, emergency alert, home network, corporate announcement, etc. The second type is a multi-BS access type in which several BSs provide an MBS service by creating one MBS zone. That is, the MBS service is provided in such a manner that the same burst is allocated from an MBS server in a plurality of cells in the same NIBS zone by using one NIBS zone identifier (ID). Therefore, service continuity and a macro-diversity gain can be obtained without a handover in case of inter-cell movement of cells including the BS.
Meanwhile, a cell in the MBS zone transmits unicast data in addition to MBS data. The unicast data is cell-specific data whose content varies from one cell to another. When the MBS data and the unicast data need to be transmitted simultaneously in one cell, the MBS data and the unicast data can be transmitted in a superposed manner. In general, the MBS service is provided by applying one fixed modulation and coding scheme (MCS) level in the MBS zone. As a result, overall performance is standardized downward in the MBS zone, which may deteriorate quality of the NIBS service.
Accordingly, there is a need for a method capable of effectively providing an MBS service.